Transcriptional activation by the pseudorabies virus immediate early protein

The pseudorabies virus immediate early (IE) protein is a potent, promiscuous activator of viral and cellular gene transcription. The promiscuous action of IE protein has led to the suggestion that it functions by an unusual mechanism. Here, we show that IE protein has the two essential features of a typical cellular activator: (1) a transcriptional activation region, and (2) a separable region that directs IE protein, or an unrelated activation region, to the vicinity of the promoter. We map the IE protein activation region to 34 amino acids, demonstrate that it is comparable in strength to the strongest known activation region, and show that it is required for the transcriptional activity of the intact IE protein. The 34-amino-acid IE protein activation region is highly acidic. We provide evidence that it uses the same cellular target as an unrelated acidic activator and a different target from that of a nonacidic activator. Our results provide insight into the function of promiscuous eukaryotic transcriptional activators.